It is known to provide a four wheel drive vehicle with a means of disconnecting two of the driving wheels when the vehicle is driven on less challenging terrain, for example “on road”. A schematic representation of a driveline 1 for such a vehicle is shown in FIG. 1A. The driveline 1 is connected to an internal combustion engine 3 arranged in a longitudinal (North South) configuration. An output shaft of the internal combustion engine 3 is connected to a gearbox 5. The disconnect means is in the form of a transfer case 7 configured to split torque between a main drive (propeller) shaft 9 and an auxiliary drive (propeller) shaft 11. The main drive shaft 9 transmits torque to a pair of rear wheels WR; and the auxiliary drive shaft 11 transmits torque to a pair of front wheels WF. The transfer case 7 is operable to split the torque delivery between the main drive shaft 9 and to the auxiliary drive shaft 11.
As shown schematically in FIG. 1B, the transfer case 7 comprise an input drive means 13, a first output drive means 15 and a second output drive means 17. The input drive means 13 is connected to an output shaft 19 of the gearbox 5. The first output drive means 15 is connected to the main drive shaft 9; and the second output drive means 17 is connected to the auxiliary drive shaft 11. The transfer case 7 further comprises transfer drive means 21. The transfer drive means 21 can comprise a drive belt or a drive chain. In alternate arrangements, the transfer drive means 21 can comprise a gear train.
The transfer case 7 comprises drive selector means for selectively engaging the transfer drive means 21 to drivingly connect the second output drive means 17 to the input drive means 13. The drive selector means can, for example, comprise a clutch mechanism 23 operable selectively to engage/disengage the transfer drive means 21. When the clutch mechanism 23 is engaged, the transfer drive means 21 drivingly connects the second output drive means 17 to the input drive means 13 The clutch mechanism 23 could be operated manually or in dependence on an electronic control signal from a transfer case control unit (not shown). The drive selector means can provide “on-demand” torque delivery, enabling the vehicle driveline 1 selectively to operate in either a two wheel drive mode (when the transfer drive means 21 is disconnected from the input drive means 13) or a four wheel drive mode (when the transfer drive means 21 is connected to the input drive means 13). A transfer case 1 incorporating drive selector means can be referred to as an on-demand transfer case. An alternative type of transfer case utilises a two-state clutch, such as a dog clutch, to connect or disconnect the transfer drive means 21 from the input drive means 13; this is referred to as a part-time transfer case.
A limitation of known on-demand transfer cases is the need to use a high torque clutch to transfer high torque loads. However, a high torque clutch typically has a high drag when operating under high delta speeds and it may not be possible to realise a highly efficient disconnect system. This can prove problematic for motor vehicles requiring a transfer case capable of withstanding high torque loads.
An alternative to providing a drive selector means is to provide a differential (not shown) in the transfer case 7. The differential engages the transfer drive means 21 in order to transmit torque from the input drive means 13 to the second output drive means 17. In use, the differential distributes drive between the front wheels WF and the rear wheels WR. The differential can additionally be in the form of a limited slip differential of which there are various different types. The operation of the differential cannot readily be controlled independently, so it is not possible selectively to engage/disengage the transfer drive means 21.
At least in certain embodiments, the present invention seeks to overcome or ameliorate at least some of the shortcomings and problems associated with the prior art arrangement described herein.